"War does not exist on other planes. It exists on your plane as a byproduct of certain challenges which the creator-entities wished to solve through materialization."
If I understand this correctly, war was selected as an experience on our plane for a specific and important reason. Seth does not divulge the reason. (Later, in Seth Speaks, he predicts that wars would vanish as a human experience near the end of this century.) However, today I stumbled upon something interesting: "Intense warfare is the evolutionary driver of large complex societies," argues Psys.org. You can read the whole article and the referenced research, but in a nutshell:
The study focuses on the interaction of ecology and geography as well as the spread of military innovations and predicts that selection for ultra-social institutions that allow for cooperation in huge groups of genetically unrelated individuals and large-scale complex states, is greater where warfare is more intense.
Perhaps the challenge was a method for causing a genetically diverse human populations to cooperate sufficiently to form "complex" states. This result could have been achieved by other means, but the "creator-entities"--knowing that life and death are, fundamentally, illusions--chose war as the most expedient path.